The present invention relates to a small fan with an electrical drive motor and an impeller coaxially disposed on the motor shaft, made to suck axially and blow with radial directional components, whose hub is turned down over the motor with its rearward broad end that is turned away from the flow, presenting an outer generated surface in rotational symmetry with respect to the motor axis, which is widened like a funnel in the direction of flow, defining the inside of an annular flow passage, in which the blades are fixed to the hub along the generated surface.
In a fan of this type known from German OS No. 19 24 611, the generated surface widens on the discharge side with an opening angle of 180.degree. (degrees). This fan has the characteristic of a radial fan and can only be used as such.
Radial fans, in many practical operating conditions, have an especially high power requirement and correspondingly cause much noise, particularly in those cases in which because of their intrinsic type the motor rpm rises in a certain part of the characteristic curve. It is precisely in the case of domestic appliances mounted in connection with sheet walls that low-noise operation is important. This is true also for the ventilation of electrical instruments, e.g. data processing facilities, disposed in more or less standardized sheet cabinets, where there are stringent requirements for low noise. Different types of instruments that are to be ventilated by the same fan, as well as different operating points of one single fan can cause very different work points to appear on the fan characteristics line of a specific fan. Such different instruments may require high pressure and yet adequate delivery from the fan on the one hand, for example, and on the other hand in free blowing operating low noise in the case of the same fan may be required. Such contradictory requirements are unsatisfactorily met by known fans.
Thus, an object of the present invention is to develop a small fan of the aforementioned type that will combine the greatest possible number of advantages of a radial fan and an axial fan.
The present invention is characterized in that the outer edges of the blades extend along a surface of rotation coaxial to the motor axis, which surface constantly widens like a funnel in the direction of flow, in that the widenings of the generated surface and the surface of rotation have an opening angle less than 90.degree. over the whole length, and are continuous, and in that the flow cross section in the flow passage between the surface of rotation and the generated surface does not narrow at any place in the direction of flow.
In small fans, motor output and therewith also the rpm depends upon the technological flow conditions of operation. This dependency is minimal over a relatively long portion of the fan characteristics line in fans according to the invention, whereby a multiplicity of possible applications is included.
As contrasted to known arrangements with radial impellers which need higher torques, the invention makes possible--aside from a reduction of drive power (or selectively a reduction of the size of the motor)--an optimal design of the motor in its efficiency because of the approximately given rpm constant over the characteristics line of the motor at this operational rpm (i.e. an additional improvements in efficiency or still further size reduction).
In the sense of noise reduction, the so-called diagonal fan of the present invention also offers advantages as contrasted to known devices, in the characteristics range that is utilized, and besides the possible power reductions in the motor can be exploited for noise reduction since a more efficient fan motor can run more slowly and hence more quietly with the same air output.
The fan according to the present invention, a so-called diagonal fan, can combine the specific advantages of axial fans (axial direction of delivery) and radial fans (axial suction flow, radial delivery, i.e. 90.degree. deflection) and thereby afford further advantages in the intermediate range between the optimal range of the axial fan and that of the radial fan.
Thus a fan according to the present invention can run at rather high pressure in spite of the same voltage with about an unchanged rpm, or over the practically used charcteristics range it can accept about constant power, and in special cases the impeller according to the invention may even be of such design that it will run faster at high pressure than when it is free blowing.
There are advantageous flow conditions if the ratio of the flow cross section at the suction end of the flow passage between the surface of rotation and the generated surface to the flow cross section at the discharge end of this flow passage is 10:10 to 10:15, and preferably 10:13.
Flow can be enhanced by the configuration and shaping of the blades, e.g. by curving them so that they are convex or concave, as seen in the direction of rotation. Flow can also be enhanced by provision of an annular wall that encloses the flow passage and extends along the surface of rotation. Such an annular wall may be either stationary or rotating. If it is to rotate, it is fixed only to the impeller. The flow is more favorably guided, especially at the suction end, if the flow passage is enclosed at the outside by an annular wall that extends along the surface of rotation and that is thickened along its suction end by a protuberance that projects radially outward.
The small fan according to the present invention can be operated to blow out freely, hence with a blowing direction that is more or less an extension of the flow passage formed between the generated surface and the surface of rotation. However, the flow can also be deflected entirely or partly in a radial direction at the blowing side. A corresponding embodiment of the present invention is characterized in that the flow passage is enclosed at the outside by an annular wall that extends along the surface of rotation and projects beyond it on the discharge side, and is widened with its discharge end to an angle of opening that is substantially greater tan that of the surface of rotation, and with an opposed stationary flat deflector wall that is perpendicular to the axis of the motor forms an annular, radially outwardly open diffusor. Any undesired turbulence on the deflector wall can be reduced by disposition of a stationary deflector wall on the discharge side behind the hub, said wall widening in the direction of flow like a funnel, coaxially to the motor axis, with an opening angle that is substantially greater than that of the generated surface.
For the drive motor, an outside rotor motor with two poled stator, advantageously an asynchronous motor, is preferred. The stator can then be made so light that, at least for the major part, it can be disposed inside the axial length of the hub, and in this way the whole axial space requirement of the fan can be reduced.
It is advantageous to strive for a ratio of the core length (1) to the air gap diameter (d) of the drive motor to 1/3 or more, especially in the case of a two-pole motor. A larger 1/d implies higher efficiency. Since however because of frequent axial limitation in construction, this is limited, and 1/d can only be increased if there is less motor power (as given in the invention, of course). A greater 1/d can be exploited in fans of the invention in a space-integrative way because the motor is weaker and therefore can have a smaller diameter. This is additionally advantageous with small rotor diameter, or it allows such diameter.
In specific cases, a fan according to the present invention, with use of an asynchronous motor for example, may have a softer characteristic, i.e. relatively high ohmic rotor resistance in free blowing at a relatively low rpm, delivering a specific amount of air, and with strong back pressure still delivering adequately and running quietly. This is managed by use of resistance-increasing alloy additives in the short circuit cage material (aluminum) of the driving rotor. The breakdown torque is then at a lower rpm, whereby the steepness of the stable range of the rpm characteristic line is less and the motor is softer, i.e. its regulatability to smaller rpm's is improved (e.g. with two pole motors, down to 1400 rpm (revolutions per minute). This additional regulatability is much desired in some cases.